The size reported in the queue is the size of the file when it was added into the queue. If the file size changes after being added to the queue it doesn't update to reflect that change. Perhaps this is the case?

no
I queued it up and didn't alter it afterwards.
But I cannot reproduce it.
I guess windows sort of stored the size while it was zipping up the file and didn't refresh the file info before I queued it up. So maybe that's why it reported a wrong file-size to FFXP. Plausible?